The present invention relates to an apparatus for reducing the risk of undesirable drifting of the parameters of an adaptive filter which is used for echo cancellation and which is coupled between a receiving branch and a transmitting branch of a four-wire loop, wherein a difference signal is generated in the transmitting branch by subtracting a compensation signal from the filter from a signal which is delivered to the transmitting branch via a two/four-wire hybrid.
In telephony systems in which adaptive echo cancellation is effected with the aid of an adaptive filter in a two/four-wire hybrid in a four-wire loop, so-called bursting may occur under certain operating conditions. This phenomenon occurs because the four-wire loop includes a feedback from the output of the adaptive filter to its input, via the far-end of the loop when damping is low at the far-end. When transmitting narrowband signals from the near-end of the loop, a correlation is therefore found between the filter input signal and the difference signal formed on the output side of the filter, even when the settings of the filter parameters are good, i.e., even when the filter has converged. This correlation can cause the filter parameters to drift in a manner which will result in the self-oscillation of the four-wire loop. When bursting occurs, the loop will self-oscillate over short periods of time, while effectively cancelling echoes over longer intermediate periods. The risk of self-oscillation is particularly manifest when transmitting periodic signals, such as sinusoidal tones, since a tone is strongly correlated with itself, even after a delay. Tones are transmitted, for instance, from modems, facsimile apparatus and in conjunction with DTMF-signalling (Dual Tone Multi Frequency).
The bursting phenomenon and a method of reducing the risk of bursting are found in:
W. A. Sethares, C. R. Johnson, Jr., C. E. Rohrs: "Bursting in Adaptive Hybrids"; IEEE Transactions on Communications, Vol. 37, No. 8, Aug. 1989, pp. 791-799; and
Z. Ding, C. R. Johnson, Jr., W. A. Sethares: "FREQUENCY-DEPENDENT BURSTING IN ADAPTIVE ECHO CANCELLATION AND ITS PREVENTION USING DOUBLE-TALK DETECTORS", International Journal of Adaptive Control and Signal Processing, Vol. 4, pp. 219-236.
The proposed method for reducing the risk of bursting uses a so-called doubletalk detector. A detector of this kind is intended to detect when signalling from a near-end subscriber is in progress, updating of the echo filter parameters being interrupted when such signalling is detected. It is difficult, however, to construct a reliable doubletalk detector. It has been found that even a comparatively weak signal from a near-end subscriber is able to greatly influence the setting of the echo filter and lead to self-oscillation. Furthermore, there is a risk that self-oscillation which has commenced for some reason or other can be detected as doubletalk. This would result in interruption of the updating of the filter parameters, and the self-oscillation would therefore persist, which is naturally unacceptable.